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Defog Breaker Tripping

  • Thread starter OriginalOwner1985z51
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OriginalOwner1985z51

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HELP! I've encountered a very weird problem on my 1985 Z51 coupe: Defog trips the associated 15-amp circuit breaker. (Late 1985 production. Early 1985 did not have this circuit breaker, which is located below the ECM, on a pigtail.)

I pulled the defog timer switch and checked it out. It runs fine on the bench. The timer runs for a full ten minutes initially, then five minutes subsequently, just as it is supposed to. It can also be turned off manually. So, the electronic part seems to be fine.

The timer's internal power control relay is also working fine. In fact, when the unit is in the car and the timer turns the relay on, too much current is flowing through the relay. It trips the 15-amp circuit breaker! When this happens, the breaker removes power from the tally lamp on the control and also from the heated mirrors and rear window, but the relay inside the timer continues to hold for the correct ten minutes. (I know this because I've measured the heavy current power source going to the timer module. It goes away when the breaker trips. I've also visually observed the relay inside the timer module and it stays held until the time has elapsed and is not burning or arcing.) The electronic portion of the timer is powered by a different feed, so this makes sense. The problem is excessive current draw in the actual defog heat circuit.

I’ve measured the resistance on the rear glass and also the rest of the circuit without the rear glass connected.

Disconnecting the harness connectors to both of the gas struts for the rear window, I see about 0.5 ohms across the rear window, including the struts. (Subtracting some extra resistance in the meter probes.) It’s the same if I meter the window directly, going around the struts, so I therefore know the struts are making good contact with the leads to the window traces, just as they are supposed to. I’ve also inspected the rear glass very carefully, using the ohmmeter, and can find no sign that anything is bridging across any of the element. I measured all around the rear window black paint and also around the latch striker insulator and I don’t seem to be getting any sneak path conductivity at those places that would tend to short across the window traces, lowering the total resistance.

Also just as there should be, there is infinite resistance from the rear window traces to ground with both struts disconnected at the harness connectors. This shows that I’m not getting any bad paths because something is grounded that shouldn’t be on the window assembly. I've even opened and closed the window with the meter connected, to make sure.

The ground wire going to the left strut measures less than 0.1 ohm to frame ground, so the ground wire appears to be solid.

As for the power wire going to the right strut, the wire measures about 3 ohms to ground with the rear strut DISCONNECTED. This also sounds about right, as in this case the measurement would include the two heated side mirrors plus the tally bulb on the control switch, all in parallel, without the rear window. When connected to the strut, it measures about 0.4 ohms, which is about what I would expect with the 0.5 of the rear window and the 3.0 of the side mirrors all in parallel.

So, I don’t see an obvious problem with the wiring, but the resulting total 0.4 ohms would draw a whopping 30 amps at 12 volts, not 15. Therefore, I am at a loss to explain what has changed to cause my 15-amp defog breaker to now be tripping. It only stays on for about 20 seconds when the defog is activated.

It’s not the breaker. I tried a new one and got the same result.

I certainly don't want to just put a higher rated breaker into the circuit. This is high current. I can just see my whole car going up in flames!

I'm even afraid to disconnect anything and then try applying power. If I disconnect the wrong thing and try to operate the system, that may provide just enough relief to keep the breaker in, but burn up something that has a problem by sending the extra current that used to go somewhere else right into the problem spot.

Therefore, I need to figure out a way to solve this without using power. I really wish I had a known good mirror element and rear window to measure. If anyone has access to such parts and a good digital ohmmeter, I certainly would be interested in your readings.

The system worked fine at one time and nothing has been changed, including the rear glass. (I know because I am the car's only owner. 20 years!)

Have these rear window traces been known to DECREASE in resistance over time? Oxidation, perhaps? Mine still seems to have that slightly brown color that I remember from when they were new. The window appears to be in perfect condition. No scratches or foreign material.

I've been doing some checking with the Helms manual for my car. It says the circuit should measure "1 ohm or less" with the defog control disconnected. That would include mirrors and rear window in parallel. Furthermore, according to the book, the wiring between the rear window and the defog control is metric #5 (AWG 10) which has a resistance of only 0.00118 ohms per foot. There can't be much more than about 10 feet of wire between the dash and the rear window strut, so the wire would add only about 0.01 ohms. In other words, the added resistance in the wire should be insignificant.

That makes sense because they would want to keep the heat out of the wire and on the window! ;-)

Anyway, a total of 1 ohm would draw 12 amps and 0.8 ohms would draw 15 amps. That means the proper total should be somewhere between those two values, since the book says "less than 1 ohm" and anything below 0.8 would draw more than 15 amps and trip the breaker.

But I'm already down to 0.5 ohms right across the rear window, with it disconnected and all by itself, just measuring across the window element! Arrrgggg!

So, it would seem the problem has to be on the rear window. But I can't see anything wrong with it whatsoever. Weird. It's driving me nuts. Could I have cleaned it with something that caused its resistance to lower? Doesn't seem likely, but I'm running out of ideas.

What's going on, here?!?!?!

Very strange.

If there are any GM pros here who have knowledge of changes that might happen to these rear windows over time resistance-wise, that would be an incredible help. Someone must have seen this sort of thing before. Any tips, ideas or observations greatly appreciated. I’m going out of my mind trying to figure this one out!!!

Despite Southern California’s sunny reputation, we do get fog, so it’s nice to have the defogger. I would hate to be without it.

I would hate to see the car burn up even more!

Many thanks.
 
Very descriptive...I've read it a few times. I measured my rear defogger but it's on a 93. It's 1 OHM exactly, disconnected from one strut of course. Is that .5 ohm reading your getting accross the back window a little low? I'm not sure what the specs are for your year.....perhaps someone else could measure their's.
 
So yours is 1 ohm? Very interesting. If mine were 1 ohm, everything would be working fine on my car, in theory, according to the way the math works out.

Before measuring, did you remember to short the meter probes together, record the resistance of the probes, then subtract that from the window measurement? The resistance in the probes becomes a big factor in these very low ohms measurements.
 
Well....the true RMS tektronix that I use requires you to short the leads in the very low ohm readings of 50 or less. I have found however that even with an older fluke DVM 83, it does very well as far as accuracy is concerned. I would wager to say at this point that the reading is in range with the true value of low resistance. I cannot explain though why yours has changed. Please let us know what you find.
 
Thanks for the info and measurement. That's a nice meter you have there! I only have a Fluke 23 (not 83), so it's much harder for me to get an accurate measurement of these very low resistance paths.

I've checked later years and also earlier years...

In 1984 and early 1985, they did not have the 15 amp breaker at all. The same circuit was protected only by the fusible link to the battery, which could obviously carry much more current than 15 amps.

In late 1985, they added the 15 amp breaker. (My car.)

Starting in 1986, just a few months later, they went to a 30 amp breaker, tapping that circuit with a separate 10 amp breaker to the heated mirrors to protect the small wiring going through the door hinges to the mirrors.

I've also received a message that someone seems to rememeber a field service bulletin on how to change the 15 amp wiring in late 1985 production to the 30 amp design that came later.

All this gives me the impression that the 15 amp design used in my car was always right on the edge. It appears that Bowling Green only built that design for a few months.

Does anyone have access to service bulletins from that long ago? Might these be online somewhere?
 

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